An internal combustion engine mounted in a vehicle includes rotating shafts such as a crankshaft and a camshaft, bearings that support the rotating shafts, and an oil pump that feeds an appropriate amount of oil to the bearings.
The oil pump has a relief valve. When a discharge pressure becomes higher than a predetermined pressure or a lift pressure (valve opening pressure), oil is released through the relief valve to adjust or keep the discharge pressure to an appropriate value.
The relief valve opens upon the discharge pressure reaching the predetermined pressure or the lift pressure, but this alone may be too simple to control the discharge pressure in some cases. Thus, a relief valve that can be opened or closed in a finer manner has been proposed to date (see, for example, Patent Literature Document 1).
The techniques disclosed in Patent Literature Document 1 will be described with reference to the following drawings.
FIG. 4(a) and FIG. 4(b) are views useful to describe a basic configuration of a conventional relief valve structure. As illustrated in FIG. 4(a), an intake port 102, a rotor chamber 103, and a discharge port 104 are formed in a pump body 101.
An inner rotor 105 and an outer rotor 106 that surrounds the inner rotor 105 are received in the rotor chamber 103.
When the inner rotor 105 is rotated directly or indirectly by a crankshaft of an internal combustion engine, the outer rotor 106 rotates along with the inner rotor 105. This rotation produces a change in the volume between the inner rotor 105 and the outer rotor 106. This change generates a pumping action composed of intake, compression, and discharge.
A valve housing 114 disposed toward the back in the drawing is further attached to the pump body 101. A first discharge portion 111, a second discharge portion 112, and a pressure relief hole 113 that each communicate with the inside of the valve housing 114 are provided in the pump body 101.
As illustrated in FIG. 4(b), a spool 115 is received in the valve housing 114 such that the spool 115 can freely move in the valve housing 114, and a valve spring 116 that urges the spool 115 in a predetermined direction is also received in the valve housing 114.
The first discharge portion 111 and the second discharge portion 112 are opened and closed by the spool 115.
The pressure relief hole 113 is a through-hole for preventing the space around the valve spring 116 from becoming tightly closed off and is thus always open.
A discharge pressure acts on a front surface of the spool 115 (a surface opposite to a back surface pushed by the valve spring 116) via a relief inflow portion 117 provided on a side opposite to where the valve spring 116 is disposed.
The spool 115 is a cylindrical body with a bottom (base portion) that opens toward the relief inflow portion 117, and a through-hole 118 penetrating in the radial direction is provided in the cylinder portion.
In FIG. 4(b), the through-hole 118 is closed by the inner peripheral surface of the valve housing 114. Thus, no oil is discharged through the first or second discharge portion 111 or 112, and the relief valve is closed.
As the discharge pressure rises, the front surface of the spool 115 is pushed, and the spool 115 moves in the direction to compress the valve spring 116. When this movement causes the through-hole 118 to coincide with the first discharge portion 111, oil on a discharge side is discharged to an intake side through the through-hole 118 and the first discharge portion 111, and the rise in the discharge pressure is mitigated.
As the discharge pressure further rises, the spool 115 further moves in the direction to compress the valve spring 116. This movement takes the through-hole 118 to a position past the first discharge portion 111. At this time, oil around the valve spring 116 is discharged through the pressure relief hole 113, and thus the movement of the spool 115 is not hindered.
The through-hole 118 becomes closed by the inner peripheral surface of the valve housing 114, no oil is discharged through the first or second discharge portion 111 or 112, and the relief valve enters a closed state. Consequently, the discharge pressure rises rapidly.
As the discharge pressure further rises, the spool 115 further moves in the direction to compress the valve spring 116. This movement takes the front surface of the spool 115 to the position of the second discharge portion 112. Then, oil on the discharge side is discharged to the intake side through the second discharge portion 112, and the rise in the discharge pressure is mitigated.
When oil is to be discharged through the second discharge portion 112, the rise in the discharge pressure may be required to be gentler. In addition, there may be a requirement for a higher accuracy in the lift pressure at which oil is discharged.
If the opening area of the second discharge portion 112 is increased to meet such requirements, in FIG. 4(a), a remaining portion 119 between the two second discharge portions 112 and 112 becomes thin, and the strength of the remaining portion 119 decreases. In addition, if the opening area of the second discharge portion 112 is increased, it becomes difficult to dispose (lay out) the second discharge portion 112.
The second discharge portion 112 is typically formed through cutting or cast coring. The cutting leads to an increased processing cost. The cast coring is advantageous in that the cutting processing cost can be saved, but the dimension precision is more likely to vary. When the dimension precision varies, it becomes difficult to stably secure the remaining portion 119, and it becomes even more difficult to increase the cross-sectional area of the second discharge portion 112. Furthermore, the cast coring leads to a variation in the lift pressure at which oil is discharged, and therefore it becomes difficult to control the oil pressure with high accuracy.
However, as there is a demand that the discharge pressure be further mitigated, a relief valve structure that allows an increased amount of oil to be discharged is being demanded.